Add subscription contract property invariant tests

[TUPM96]

Resolves #402

Summary:

• Add a real Soroban integration property/invariant test target for the subscription contract using the storage contract and generated clients.
• Cover plan-count consistency, repeated charge accounting, lifecycle state transitions, refund bounds, and subscriber/accounting invariants.
• Update the invariant workflow to run the registered package-level property_invariants target with CI-controlled proptest cases and shrink iterations.
• Apply rustfmt to existing contract files so the workflow format check passes cleanly.

Verification:

• cargo fmt --check
• PROPTEST_CASES=16 PROPTEST_MAX_SHRINK_ITERS=1000 cargo test -p subtrackr-subscription --test property_invariants -- --nocapture
• cargo test --workspace --lib --verbose
• git diff --check

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